1. Field of the Invention
The present invention relates to a recording apparatus for conducting recording on a recording medium, and more particularly, to a so-called serial type recording apparatus for conducting recording by the movement of a recording head.
2. Description of the Related Art
FIG. 1 shows an example of a conventional recording apparatus.
The recording apparatus shown in FIG. 1 is of the ink jet type. An ink jet recording head 101 is mounted on a carriage 102 which is capable of moving back and forth in the direction indicated by an arrow in FIG. 1. The carriage 102 is connected through its connection portion 102A to a driving belt 106 extending between a driving pulley 104 fixed to a rotary shaft of a driving motor 103 and an idler pulley 105 disposed on the side of the apparatus remote from the driving pulley 104. Through this connection the carriage 102 is moved back and forth along a sliding rail 107 in accordance with the forward and reverse rotations of the driving motor 103. During the movement of the carriage 102, the recording head 101 conducts recording by ejecting ink onto a sheet of recording paper 108 which is conveyed by means of a convey roller 109.
In the manufacture of the driving and idler pulleys in the above-described recording apparatus, eccentricity or imperfect roundness is unavoidable, given the today's manufacturing technologies. In the conventional recording apparatus, this eccentricity or the like may generate nonuniformity in the speed at which the driving belt 106 travels, which leads to generation of recording irregularities that cannot be ignored in a case where the degree of eccentricity or non-circularity exceeds a predetermined limit. In the case of a printer with a single recording head, such as that shown in FIG. 1, i.e., a printer for recording in a single color, such recording irregularities are not often so readily noticeable and are thus acceptable.
In recent years, as color display of personal computers or the like has become popular, there has been an increasing demand for printers capable of color recording. In such printers, recording heads 101A to 101D, each corresponding to one of a plurality of colors, are mounted on the carriage 102, as shown in FIG. 2. These colored inks are placed on top of one another to form, for example, a single pixel, by which full-color recording is performed. Generally, recording heads for four colors, cyan, magenta, yellow and black, are provided in full-color recording. In the case where colored inks are placed on top of one another, the above-described recording irregularities appear in the form of color misalignment, which is relatively clearly noticeable to a viewer.
Recording irregularities and color misalignment, caused by an eccentric pulley, will now be described below.
FIG. 3 schematically shows how the driving belt 106 is moved by the rotation of the driving and idler pulleys 104 and 105 which are completely round and which are not eccentric. When the driving pulley 104 rotates 180 degrees from position `a` to position `b`, the point on the driving belt 106, located at position `A`, moves to position "B". With rotation of the driving pulley by another 180 degrees, the point on the driving belt 106 further moves to position `C`, whereby one revolution of the driving pulley 104 is completed.
At that time, if the pulleys are not eccentric, the distance between positions A and B is equal to the distance between positions B and C, and the speed at which the driving belt 106 travels is maintained constant when the driving pulley 104 rotates at a constant speed. Hence, an image having no color misalignment can be recorded by means of the recording heads which conduct recording at predetermined time intervals.
Next, the case in which the driving pulley 104 is eccentric in an amount of `.alpha.` will be described with reference to FIG. 4.
First, it is assumed that the driving pulley 104 is located at the position indicated by a solid line in FIG. 4. The driving belt 106 is wound around the pulley 104 past the position indicated by `al`, and position A shown in FIG. 3 is shifted to the right when compared with the case when the pulleys are completely round and not eccentric. In this case, the actual length of the belt between the two pulleys varies, and tension of the belt thus varies, thereby generating elongation or compression of the belt or displacement of the portion which rotatably supports the pulley. Based on the experimentation, the amount of the above-noted shift is approximately .alpha./2.
When the driving pulley 104 rotates 180 degrees from the above-described position, the driving belt 106 is wound around the pulley 104 past the position indicated by `bl`. At that time, a point on the belt located at position `A`, which is supposed to reach position `B`, as shown in FIG. 3, moves due to the eccentricity to a position shifted from normal position `B` to the left by .alpha./2. When the driving pulley 104 rotates another 180 degrees and thus makes one complete rotation, a point on the belt located at position `A` moves to a position shifted from normal position `C` to the right by .alpha./2.
FIG. 5 is a positional discrepancy diagram, in which the ordinate axis represents positional discrepancy from the normal position of the belt obtained when eccentricity does not exist (rightward discrepancy has a positive sign), while the abscissa axis represents the normal position at which the belt is supposed to be located.
The comparison made between the results of the recording conducted using the belt which generates positional discrepancy and the results of the recording conducted using the belt which is free from eccentricity (this recording being hereinafter referred to as normal recording) is shown in FIG. 6. FIG. 6 schematically shows the positions at which ink dots are formed in normal recording and in the recording conducted using the eccentric pulley. As is clear from FIG. 6, non-uniformity in the recording density occurs in accordance with the rotational period of the driving pulley 104 in the recording conducted using the eccentric pulley.
The positional offset which occurs when the recording apparatus employing a plurality of recording heads is used (two heads for the convenience of explanation) will be described below with reference to FIG. 7.
It is assumed in the apparatus shown in FIG. 7 that two heads 101A and 101B are mounted on the carriage 102 apart from each other by a distance l, that the distance through which the carriage 102 is moved by one revolution of the driving pulley 104 is L (which is equal to the length between point A and point C on the sheet of recording paper 108), and that the relationship between l and L is expressed by l=L/2. When the pulley is not eccentric, the head 101A ejects ink to form a dot when it reaches point A as a consequence of rotation of the driving pulley 104. Thereafter, the head 101B ejects ink when the driving pulley 104 further rotates and the head 101A thereby reaches point B, by which dots can be formed on top of each other at the same point A.
However, in a case where eccentricity .alpha. is present on the driving pulley 104, the dots recorded by means of the head 101A shift from their regular positions, and recording shown in FIG. 6 is thus conducted. At that time, the positions of the dots formed by means of the head 101B also shift. The phase of the positional discrepancy generated by the head 101B is different from the phase of the positional discrepancy generated by the head 101A by a distance l between the two heads, as shown in FIG. 8.
That is, since the distance l between the two heads is equal to one half of the distance through which the carriage 102 is moved by one rotation of the pulley (the distance between points A and B), the head 101B ejects an ink at the instance the driving pulley 104 has rotated by 180 degrees after the ejection of the ink from the head 101A. Hence, the positional discrepancy of the dot formed by the head 101B is equal to the positional discrepancy of the dot recorded by the head 101A at point `B`. In other words, the positional discrepancies generated by the heads 101A and 101B are out of phase by the distance l.
At point `A`, the dot formed by the head 101A is offset from the dot formed by the head 101B by `.alpha.` since discrepancy of the dot formed by the head 101A is +.alpha./2 and discrepancy of the dot formed by the head 101B is -.alpha./2. In a case where the amount of eccentricity of the driving pulley is 30 .mu.m, the positional offset which occurs at point `A` is 30 .mu.m. In practice, the positional offset is also affected by the eccentricity of the idler pulley 105 and thus tends to be increased.
The pixel density of the available full-color printers is, in general, 360 to 400 dots per inch, which is 70.6 to 63.5 .mu.m in terms of the distance between the adjacent dots. When the aforementioned positional offset occurs in such a printer, characters may be recorded in slightly different tints or may appear in a blurred state.